Geed Lab

Neuroplasticity and Motor Function Recovery after Stroke

Neural basis of involuntary shoulder-hand coupling

A subset of individuals with stroke experience uncontrolled, involuntary flexion at elbow and wrist-hand flexor muscles when the shoulder is elevated or abducted, clinically referred to as a pathological muscle synergy. What is the neurophysiological basis for these involuntary muscle activations in individuals with stroke?  Clinicians have recognized pathological synergies like the flexor or extensor synergies for a long time, but we do not fully understand the neural basis for why they manifest after stroke.

Why is it important to study why and how pathological synergies manifest? Pathological synergies cause significant impairment and limitations in productive use of the upper extremity. Patients can grasp objects, but releasing objects is  difficult because they cannot open their hand. Reaching for objects in the workspace is particularly difficult, which causes significant loss of independence in activities and instrumental activities of daily living, like dressing, bathing, feeding, or engaging in recreational activities.

What are we doing to study mechanisms underlying pathological synergies?
Our lab (and others) have hypothesized several potential mechanisms. For a start, we are testing, if changes in shoulder position affects the excitability of cortical neurons that project to hand flexor and extensor muscles. Using TMS, we test healthy controls and individuals with stroke to test how changes in shoulder positions affect hand motor control. Our studies showed that healthy controls show increased cortical excitability of neurons that project to wrist-digit extensor muscles when the shoulder is abducted, which makes it easier to open the hand as happens when we prepare to grasp during reach. This may reflect a hardwired mechanism to coordinate shoulder and hand joints for prehension. Individuals with stroke seem to lose this coupling, the "extensor assist" on shoulder abduction.

In a second step, we are testing how progressive increases in activation of shoulder muscles (that elevate or abduct the joint) affects excitability of the neurons that project to hand flexor and extensor muscles.


Neuronal Correlates of Functional Coupling between Reach- and Grasp-Related Components of Muscle Activity

Shashwati Geed, M. L. McCurdy, P. V. van Kan

Front. Neural Circuits, 2017

Shoulder position and handedness differentially affect excitability and intracortical inhibition of hand muscles

Shashwati Geed, Megan Grainger, M. Harris-Love, P. Lum, A. Dromerick

Experimental Brain Research, 2021


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